Metabolic interfaces between growth and reproduction. V. Pulsatile luteinizing hormone secretion is dependent on glucose availability.

To test the hypothesis that mechanisms controlling the secretion of LH are modulated by glucose availability, the acute effects of glucoprivation were studied. The model was the gonadectomized male lamb raised on a limited diet of artificial milk. The approach was to monitor LH secretion before and after the administration of a competitive antagonist of glucose metabolism, 2-deoxyglucose (2DG). We first determined whether LH secretion was influenced by glucose availability by administering 2DG at several doses. Peripheral administration of the glucose antagonist (240 and 480 mg/kg 2DG, single iv injection) transiently decreased LH pulse frequency, but not LH pulse amplitude. By contrast, LH secretion (frequency or amplitude) was not affected by lower doses (60 or 120 mg/kg) of the glucose antagonist. A second study was conducted to determine whether either the pituitary gland or the GnRH neurosecretory system per se is directly affected by short term glucoprivation. The competency of the pituitary was assessed by administering GnRH during the time when LH secretion is suppressed by pharmacological glucose blockade. Similarly, the function of the GnRH neurosecretory system was assessed by administering a GnRH secretagogue (N-methyl-D,L-aspartate) under the same glucoprivic conditions. In response to an optimized iv dose of 2DG, LH pulse frequency decreased. However, in lambs that received either GnRH or N-methyl-D,L-aspartate during the period of glucoprivation, LH pulse frequency was sustained at levels comparable to those before 2DG was given. To determine whether the effect of glucoprivation was central in origin, the glucose antagonist was administered into the lateral cerebral ventricle at 1/100th the doses used peripherally. Central administration of 2DG, independent of dose, transiently decreased LH pulse frequency, but not pulse amplitude. However, unlike the case with peripheral injection, plasma glucose values did not change after the administration of any dose of 2DG tested centrally. These findings indicate that glucose availability in the developing sheep influences LH secretion. Moreover, based upon analysis of LH pulse frequency, glucoprivation does not directly impair either the pituitary gland or the GnRH neurosecretory system. Collectively, these results suggest that glucose availability affects LH secretion by acting within the central nervous system at a detection site(s) peripheral to the GnRH neuron.

Free fatty acids suppress growth hormone, but not luteinizing hormone, secretion in sheep.

An experiment was conducted to determine the effects of exogenously administered FFA on GH and LH secretion in sheep. Ovariectomized ewes received iv infusions of a mixture of FFA (166 mg/min; n = 5) or 0.9% saline (n = 4) for 10 h. Jugular blood was sampled every 15 min for 14 h, beginning 4 h before initiation of infusion. After 8 h of FFA or saline treatment, each ewe received a pituitary challenge of 10 micrograms GRF and 1 microgram GnRH, administered together as an iv bolus. Lipid infusion increased (P less than 0.01) serum FFA concentrations to levels characteristic of those in fasted sheep [23.0 +/- 0.8 mg/100 ml (mean +/- SE)]. Frequency of GH pulses (P less than 0.01) and the GH response to GRF (P less than 0.0001) were suppressed by FFA treatment. Mean serum GH concentrations increased gradually (P less than 0.01) during the 10-h infusion period in saline-treated but not lipid-treated, ewes. This finding may reflect diurnal changes in somatotrope secretory activity that are blocked by FFA. Mean serum LH concentrations, LH pulse frequency and amplitude, and the LH secretory response to GnRH were unaffected by FFA or saline infusion. In agreement with previous work in sheep and other species, these results provide evidence for an inhibitory effect of FFA on GH release. The exact mechanism responsible for this action, however, remains to be elucidated. Finally, acutely elevated FFA levels do not appear to influence LH secretion in the ovariectomized ewe.

Plasma catecholamines in the chronically cannulated sheep fetus: predominance of L-dihydroxyphenylalanine.

Dihydroxyphenylalanine (Dopa) and catecholamines (CA) concentrations were determined in plasma collected from chronically cannulated fetal and maternal sheep from days 125-140 of gestation. Dopa was measured by a radioenzymatic assay, followed by ion exchange chromatography and high performance liquid chromatography. The assay was linear to 2.5 ng Dopa, and its sensitivity was 35-45 pg. Dopamine (DA), norepinephrine, and epinephrine were determined simultaneously by the same radioenzymatic incubation procedure, followed by solvent extraction and two-dimensional thin layer chromatography. The Dopa level in the fetal circulation was 10-25 times higher than that of DA, 5-10 times higher than that of norepinephrine, and 100 times higher than that of epinephrine. Furthermore, Dopa was the only CA that was significantly (P less than 0.05) higher in fetal (3.5-4.5 ng/ml) than in maternal plasma (1-1.5 ng/ml). The data indicate that Dopa is the predominant circulating CA in the sheep fetus. While the physiological importance of this observation is unknown at the present time, fetal Dopa might serve as the source of free DA in fetal urine and/or amniotic fluid.

Effects of free fatty acids on luteinizing hormone and growth hormone secretion in ovariectomized lambs.

The effects of FFA on circulating LH and GH concentrations in ovariectomized ewe lambs were investigated. Lambs (n = 14) were weaned at 2.5 months, ovariectomized at 6.5 months, and used at 8.5 months of age. From weaning until day 0 of the experiment, lambs were fed to maintain body weights (23 kg). On day 0, serum FFA concentrations and mean serum LH concentrations and number and amplitude of LH pulses, as assessed in blood samples collected every 12 min for 4 h, were 6.4 +/- 0.6 mg/100 ml, 0.57 +/- 0.08 ng/ml, 0.45 +/- 0.09 pulses/h, and 0.73 +/- 0.11 ng/ml, respectively. Double the maintenance feeding, beginning day 1, increased (P less than 0.01) body weights by 16% and LH pulse frequency by 82%, but had no effect (P greater than 0.1) on FFA concentrations, mean LH concentrations, or LH pulse amplitude by day 14. On day 14, lambs were infused with lipid (n = 9; 95.8 mg/min) or 0.9% saline solution (n = 5) for 8 h. Blood samples were collected at 12-min intervals for 12 h, beginning 4 h before infusions. FFA levels increased (P less than 0.01) in lipid-infused animals to 27.6 +/- 2.9 mg/100 ml by 4 h of infusion. Mean LH concentrations and LH pulse frequency and amplitude were unaffected (P greater than 0.1) by treatment. In contrast, mean GH concentrations and GH pulse frequency, which were similar (P greater than 0.1) between groups before infusion (14.0 +/- 0.8 ng/ml and 0.36 +/- 0.07 pulses/h, respectively) were decreased by FFA treatment by 51% (P less than 0.01) and 81% (P less than 0.006), respectively. GH pulse amplitude was highly variable and unaffected (P greater than 0.1) by treatment. In summary, elevated FFA levels appear to inhibit the release of GH, but not LH, in the ovariectomized ewe lamb.

N-methyl-d, l-aspartate stimulates growth hormone but not luteinizing hormone secretion in the sheep.

The objective of this experiment was to determine the effects of N-methyl-d, l-aspartate (NMA) on luteinizing hormone (LH) and growth hormone (GH) secretion in castrated male sheep. Blood was sampled from Hampshire wethers every 15 min for 8 hr on day 1. At 4 and 6 hr after the initiation of the experiment, wethers were treated i.v. with NMA at a dose of 12 mg/kg body weight (n = 5) or .9% saline (n = 5). The dosage of NMA was within the range of doses that was previously demonstrated to stimulate LH secretion in monkeys. Blood samples were also collected every 15 min for 1 hr on day 2, beginning 24 hr after the first injection of NMA or saline. Treatment with NMA had no effect on mean LH concentrations, LH pulse frequency or LH pulse amplitude during the 4 hr period following the first injection on day 1. On day 2, however, mean LH concentrations were lower (p less than .01) in NMA versus saline-treated wethers. Conversely, administration of NMA evoked a dramatic increase (p less than .02) in mean GH concentrations on day 1. The mechanisms responsible for the effects of NMA described herein and whether or not these effects are relevant to the physiological control of LH and GH release in the sheep warrants further scrutiny.

Effects of cysteamine on pulsatile growth hormone release and plasma insulin concentrations in sheep.

The effects of cysteamine (CSH; 0, 50, or 100 mg/kg BW), a somatostatin depleting agent, on growth hormone (GH) and insulin (INS) secretion were studied in sheep (Ovis aries). Cysteamine was administered as a single intragastric bolus on day 0 (0900). Jugular blood samples were collected at 15-min (GH) and 2-hr (INS) intervals over an 8-hr period (1100-1900) on day 0, 3, and 7. Intragastric administration of CSH at 50 mg/kg BW augmented (quadratic, P = .04) mean plasma GH concentration, with the greatest response occurring on day 3. Baseline GH concentrations were elevated in wethers dosed with 50 mg/kg BW CSH on day 3, whereas wethers dosed with 100 mg/kg BW CSH had lower baseline GH concentrations on day 0 (CSH x day interaction, P = .02). Cysteamine administration increased GH pulse amplitude (quadratic, P = .15), with the greatest magnigtude of change occurring with 50 mg/kg BW CSH on day 0 and 3. Frequency of GH pulses was increased (quadratic, P = .10) following CSH treatment. Administration of 100 mg/kg BW CSH augmented plasma INS on day 0 (CSH x day interaction, P = .09). These findings indicate that CSH alters GH and INS secretion in a dose-dependent and temporal manner. The observed changes in mean and baseline plasma GH concentrations associated with 50 mg/kg BW CSH are consistent with somatostatin depletion; however, higher doses of CSH appear to disrupt GH secretion by an alternative mechanism.

Luteal phase deficiencies in peripubertal rhesus monkeys: mechanistic considerations.

Short luteal phases in peripubertal monkeys were identified by abbreviated progesterone secretion. Evaluation of the hormonal profiles for LH, FSH, and estradiol revealed that depressed serum concentrations of FSH during the follicular phase and LH and estradiol during the luteal phase were associated with luteal defects. In addition, morphological differences in the corpora lutea of short and normal luteal phases were identified. A mechanism is presented whereby inadequate follicular phase FSH secretion results in abbreviated luteal function.

Castration increases pulsatile luteinizing hormone release, but fails to diminish mounting behavior in sexually experienced bulls.

We tested the hypothesis that mounting and chemoinvestigatory behaviors are testosterone-dependent in bulls. Eighteen bulls were divided into three treatment groups: intact (I), castrated (C) and castrated+testosterone (T). Sexual behaviors of all bulls were tested with an unrestrained receptive female 1 week prior to and weekly for 4 weeks after castration. Mounts with intromissions, aborted mounts and flehmen responses were quantified for each test period. In addition, patterns of LH and testosterone secretion were assessed at these times. Neither mounts with intromissions nor aborted mounts were affected by treatment. In contrast, numbers of flehmen responses were lower in C bulls than in the other groups following castration. Before castration, concentrations of LH were not different among groups and LH pulse frequency was approximately one pulse per hour. Castration resulted in a 2-fold increase in mean concentrations of LH and a 6-fold increase in LH pulse frequency. Neither mean concentration of LH nor LH pulse frequency changed in I or T bulls. The data fail to support the hypothesis that mounting behavior is T-dependent, but supports the hypothesis that this steroid hormone regulates flehmen behavior in sexually experienced bulls.

Influences of month of birth and age on patterns of luteinizing hormone secretion in prepubertal heifers.

An experiment was done to determine if month of birth and age influenced patterns of luteinizing hormone (LH) secretion in prepubertal heifers. Mean concentrations of LH increased linearly (P<.05) in March-born heifers between one and seven months of age. This was partially due to an increase in number of LH pulses. The prepubertal pattern of LH concentrations was quadratic (P<.05) for heifers born in September because concentrations were slightly higher (P=.15) than those in March-born heifers at one month of age. There were no differences between groups during the remainder of the prepubertal period (3 to 7 months). There was a tendency (P=.18) for September-born animals to reach puberty at younger ages than those born in March. September-born heifers also had greater (P=.06) average daily gains, but body weights at puberty were similar for the two groups. These results show that season of birth influenced LH concentrations at one month of age, but did not significantly affect the increase between three and seven months of age.

Estrogen - Induced release of luteinizing hormone in prepubertal and postpubertal heifers.

This experiment was designed to determine the age at which estradiol-17beta (E(2)) first induces a preovulatory-like surge of luteinizing hormone (LH) in prepubertal heifers. Responses of prepubertal animals 3 to 4 and 5 to 6 months of age were compared with those of postpubertal heifers that received 25 mg prostaglandin F(2)alpha at 0800 hr on day 15 of the estrous cycle. E(2) (500mug) induced surges of LH in 1 5 heifers 3 to 4 months of age, 3 3 heifers 5 to 6 months of age and 5 5 postpubertal heifers. Duration of response and interval between E(2) injection and peak of the response were longer in postpubertal heifers than in those 5 to 6 months old (P<0.10). Peak response and total amount of LH released were greater in animals 5 to 6 months old (P<0.10). Only one prepubertal heifer had elevated concentrations of progesterone following an LH surge. Four of 5 postpubertal heifers receiving E(2) and 3 of 4 postpubertal heifers receiving corn oil had corpora lutea and similar patterns of progesterone concentrations. We conclude that ability to release an LH surge in response to E(2) develops in heifers between 3 and 5 months of age, but that this induced surge does not cause ovulation.

Change in morphology of corpora lutea, central luteal cavities and steroid secretion patterns of postpartum suckled beef cows after melengestrol acetate with or without prostaglandin F2alpha.

Change in morphology of the corpus luteum (CL) and patterns of progesterone and estradiol secretion after treatment with melengestrol acetate (MGA) were monitored in postpartum beef cows. Twenty Angus cows were randomly assigned to MGA or MGA + prostaglandin F(2alpha) (PGF) treatments. All cows were fed 0.5 mg of MGA per cow per day for 14 d. The MGA-treated cows (n = 10) were allowed to return to estrus spontaneously at the second estrus after withdrawal of MGA from the feed. The MGA + PGF-treated cows (n = 10) received an injection containing 25 mg of PGF(2alpha) 17 d after the last feeding of MGA. Cycle 1 was defined as the first luteal phase after MGA feeding and Cycle 2 represented the subsequent cycle or luteal phase after PGF. Blood sampling and transrectal ultrasonography of the ovaries was done daily through the completion of 2 estrous cycles upon removal of MGA from the feed. Blood samples were analyzed for plasma progesterone and estradiol concentrations. Area of CL and fluid-filled cavities within each CL were determined by ultrasonography. Concentrations of progesterone and area of CL were similar between cycles and treatments. Estradiol concentrations were higher (P < 0.05) in Cycle 2 than in Cycle 1. Fluid-filled cavities were larger (P < 0.001) in Cycle 1 than in Cycle 2 for both mid-luteal (Days 5 to 9) and late-luteal (Days 10 to 14) phases. Multiple CL (2 or more during 1 cycle) were observed in 5 cows. Progesterone concentrations and total area of luteal tissue did not change with respect to treatment or cycle, but CL morphology was altered in the first cycle after MGA treatment. Of the 19 cows that ovulated after withdrawal of MGA, 3 experienced a short luteal phase. These data characterize changes that occur among cows that are fed melengestrol acetate during the postpartum period and enhance observations from prior studies regarding MGA use.

Effects of adrenalectomy on serum concentrations of progesterone during the luteal phase of sheep.

An experiment was conducted to test the hypothesis that the adrenal gland influences luteal activity in sheep. Twelve Finnish Landrace x Southdown ewes were either bilaterally adrenalectomized (n = 6) or sham adrenalectomized (n = 6) during the breeding season. At approximately 37 and 47 d after surgery, all ewes received intramuscular injections of cloprostenol to synchronize estrus. Blood samples were taken via jugular venipuncture at 48-h intervals between 1 and 19 d after the last cloprostenol treatment. Serum concentrations of progesterone were determined in each of these samples. Analysis of variance showed that concentrations of progesterone during the luteal phase were lower (P<0.05) in adrenalectomized ewes than in sham-operated controls, but that patterns of progesterone were similar for both groups of sheep. Based on these results, we conclude that the adrenal gland does not appear to be necessary for initiation of luteal regression in ewes.

Estrus synchronization with an oral progestogen prior to superovulation of postpartum beef cows.

Ovarian follicular dynamics and steroid secretion patterns were monitored in postpartum beef cows that were synchronized for estrus with melengestrol acetate (MGA) or prostaglandin F(2alpha) (PGF) prior to superovulation. Twenty-four muhiparous Angus cows were stratified by number of days postpartum to an MGA or PGF treatment prior to superovulation. Cows in the MGA group were fed 0.5 mg MGA/d for 14 d in a grain carrier. Superstitnulatory treatments began 14 d after withdrawal of MGA from feed or 11 d after administering a single injection of 500 microg cloprostenol (PGF). Supersthnulatory treatments (FSH) were administered twice daily in decreasing doses (7.5, 5, 5, 2.5 mg) over 4 d. Sixty and 72 h after initiating the superstimulatory treatments, all cows were treated with 750 microg and 500 microg PGF, respectively Cows were inseminated at 0, 12, and 24 h from the onset of standing estrus with semen from 2 proven sires. Cows within treatment were inseminated with 1, 2 and 1 (single) or 2, 4 and 2 units (double) of semen at the designated insemination times. Blood sampling and transrectal ultrasonography of ovaries were performed daily beginning 2 d prior to the initiation of FSH treatment and were continued through embryo recovery. Ovaries were examined daily to determine the number and size of follicles. Plasma samples were analyzed for progesterone and estradiol. Follicles were counted and categorized based on a 5 to 9 mm range or >/= 10 mm. At the end of superovulatory treatment there were more (P /= 10 mm among cows that were estrus synchronized with MGA (75 +/- 1.2) than with PGF (3.9 +/- 1.2) These differences were reflected in higher (P

Effects of dietary energy on control of luteinizing hormone secretion in cattle and sheep.

Prolonged restriction of dietary energy delays onset of puberty, disrupts cyclicity in sexually mature animals, and lengthens the postpartum anestrous period in domestic ruminants. One important mechanism by which energy restriction impairs reproductive activity seems to be suppression of the increase in LH pulse frequency that is necessary for growth of ovarian follicles to the preovulatory stage. Under-nutrition apparently inhibits pulsatile secretion of LH by reducing LHRH secretion by the hypothalamus. The ability of an animal to sustain a high-frequency mode of pulsatile LH release is related to its metabolic status. Mechanisms linking metabolic status to LHRH secretion have not been fully characterized. Changes in body fat have been associated with changes in reproductive activity, but it is unlikely that body fat per se regulates LHRH secretion. It is possible that pulsatile LHRH release is regulated by specific metabolites and(or) metabolic hormones that reflect nutritional status. Alternatively, availability of oxidizable metabolic fuels, such as glucose and nonesterified fatty acids, may influence activity of neurons that control LHRH release. Our understanding of how the central nervous system transduces information about nutritional status into neuroendocrine signals that control reproduction in cattle and sheep is limited by a lack of information concerning the nature of neurons controlling LHRH release in these species.

Teaching animal science: education or indoctrination?

Traditional animal science curricula ignore sociological aspects of scientific research and therefore portray scientific knowledge as value-free. This view gives rise to a teaching method that involves imparting lists of scientific facts that are to be accepted by students without critical evaluation. This amounts to little more than indoctrination and misrepresents science as a system of knowledge. An alternative approach is based on the view that science is a creative human activity that reflects the values and biases of its practitioners. The goal of this approach is to teach students to think analytically and to make independent judgments about scientific claims. This requires a scientific literacy: an understanding of principal scientific theories, the nature of scientific research, and the relationship between science and society. To achieve this goal, a teacher must become less of an authority figure, whose role is to simply pass on information, and more of a facilitator, whose role is to promote questioning, exploration, and synthesis. This requires a learning community in which students feel comfortable taking risks and develop the courage to make and defend judgments. This teaching approach enhances the intellectual and ethical development of students, allowing them to serve themselves and society in responsible ways.

Toward a pluralistic animal science: postliberal feminist perspectives.

Women have not had the authority to define the conditions of animal agriculture or to develop strategies for addressing its problems because the field of animal science has been and remains dominated by men. Therefore, criteria for assessing the professional performance of animal scientists reflect the views of men and the experiences of masculinity. Recent attempts to improve the retention rates of women in the animal science profession have focused on socializing female protégés to the norms and expectations of the discipline. This approach is based on traditional liberal beliefs that all humans are essentially alike and that embodiment and(or) social location are irrelevant in understanding the world. Such socialization assumes that problems of gender inequity can be solved by providing opportunities for women to succeed in the existing system, with its existing norms. This approach ignores the possibility that the structure of women's lives and the normative commitments arising from women's experiences may be quite different from those of men. Professional reward structures that favor the life experiences and perspectives of men and ignore those of women coerce women into adopting values that may be incompatible with their identities. This not only devalues their perspectives and contributions, it forces them to commit self-betrayal. This is both harmful and antithetical to the idea of an empirically relevant science that blends multiple perspectives and understands diverse values. Substantive diversity in the animal science community requires standards that reflect and accommodate the pluralism in which we live.

An appropriate role for ethics in teaching contemporary issues.

There seems to be general agreement among animal scientists that training in contemporary issues is an appropriate goal of animal science curricula. One approach is based on the view that public policy issues embody moral dilemmas that arise when the moral perspectives (values) of opposing groups are incompatible. Based on this view, an understanding of such issues requires some training in ethics. More specifically, knowledge of major moral theories and the nature of values is necessary to identify and analyze the moral components of issues. An appreciation for ethics also provides insight into addressing issues in a morally responsible manner. The plurality of values and the contextual nature of policy issues precludes solutions based on application of universal moral principles. Nevertheless, informed judgments, based on the collective wisdom of individuals, frequently offer solutions that address opposing values. Such judgments can be attained when those affected by issues form communities, attempt to achieve a collective understanding of the problem, and, if possible, construct a common set of values that facilitates consensus. Based on this analysis, issues courses should facilitate the development of learning communities wherein students and teachers critically analyze (i.e., deconstruct) and reconstruct issues in ways that enhance understanding and instill a respect for diverse moral perspectives. An expression of such understanding and respect is the ability to distinguish between situations when consensus offers fair solutions and when it subjects minority groups to the tyranny of the majority.

Critical perspectives of animal agriculture: introduction.

The American Society of Animal Science has recently focused its attention on a variety of contentious issues in animal agriculture. This paper deals with critique, a philosophical approach to analyzing and understanding issues. This method has been employed by various contemporary philosophers. For example, feminist theorists have used this approach to critically analyze sexual harassment. Critique involves a critical analysis of the discourse (ideas or language) and practices that define the social reality in which we live. How we think about the world and how we behave in it determines how we humans interact with each other as well as with the rest of nature. This social structure is associated with power structures that benefit some individuals and harm others. In this paper, I demonstrate how critique can be used to better understand the social reality of animal agriculture. By analyzing certain popular texts in this field, I show that a "mechanical view of nature" is dominant in animal agriculture and argue that such a view contributes to a social reality that can be harmful to some humans and other animals. I conclude that various contentious issues can be better addressed when we engage in a critical analysis of this conceptual framework and base our analysis on the experiences of many different people, including those who have been harmed by our current system of animal agriculture.

Postmodernism for animal scientists.

Many scientists regard the term "postmodernism" as controversial. Because postmodern theorists question whether science can be objective, some scientists view postmodernism as anti-scientific. In this paper, we argue that traditional accounts of science developed during the modern era (16th, 17th, and 18th centuries) are still influential in animal science, but are no longer plausible. In particular, the view that science automatically leads to human betterment seems to be disingenuous. A postmodern view that portrays science as a political activity seems more plausible, and offers a means to better understand contentious policy issues that involve science. Although most animal scientists accept the view that theory selection, experimental designs, and technology development require value-laden judgments, most fail to recognize that such values may be politically motivated and embrace prevailing political structures. Postmodernists such as Michel Foucault argue that through the generation of knowledge, scientific disciplines create a discourse that serves to maintain a particular social structure that has political implications. Viewed in this way, it becomes clear how various interest groups can be critical of certain scientific programs. For example, groups that oppose research dealing with cloning, genetically modified organisms, and intensive livestock production may not be as much opposed to science as they are to the political interests served by this science. In other words, such groups view these research agendas as promoting policies that place them at risk. Such a postmodern account of science, may help animal scientists better understand the nature of contentious issues, and provide a basis for reforming the animal science discipline in ways that make it more responsive to the diverse interests of a pluralistic society.

Effects of melengestrol acetate on reproductive behavior and concentrations of LH and testosterone in bulls.

We investigated the use of an orally active progestin (melengestrol acetate; MGA) to suppress reproductive activity in yearling beef bulls. Twenty-four crossbred bull calves were given a daily dose of 0, 0.5, 1.0, or 2.0 mg MGA for 99 d. Pulsatile patterns of LH and concentrations of testosterone and MGA were characterized on d 8, 36, 63, and 92 of the experiment. Numbers of aborted mounts, mounts with intromission, total mounts, and flehmen responses were assessed on d 15, 43, 71, and 99. Plasma concentrations of MGA were proportional to dose of MGA. Melengestrol acetate did not consistently affect mounting behavior in a dose-related manner, but, on d 99, number of total mounts for MGA-treated bulls was lower (P = 0.07) than that for control bulls. On d 15, MGA suppressed (P = 0.07) numbers of flehmen responses in a dose-dependent manner, but this effect was not sustained throughout the experiment. On d 8, concentrations of testosterone in control bulls were higher (P = 0.02) than in MGA-treated bulls, but this effect was not observed at other time periods. Overall, MGA caused slight decreases in mean concentrations of LH (P = 0.09) and LH pulse frequency (P = 0.06). Scrotal circumference was not affected by MGA. None of the behavioral traits was correlated with mean concentrations of LH or LH pulse frequency. Mounting activity was not correlated with testosterone concentrations, but number of flehmen responses was positively correlated with testosterone concentrations (P = 0.01). These results fail to support the hypothesis that progestins impair male sexual behavior or fertility in males.